Tire treads for pneumatic tires typically have running surfaces of a singular rubber composition and consistent physical properties across the face of the tread intended to be ground contacting.
Often the tire tread may be of a cap/base construction composed of an outer tread cap layer presenting the running surface of the tire and an underlying tread base layer as a transition between the tread cap layer and the tire carcass. The tread cap layer itself may be of a lug and groove configuration with the outer surface of the lugs, including lugs in a from of ribs, themselves presenting the running surface of the tire tread. Such overall tire tread cap/base construction is well known to those having skill in such art.
For example, an all-season tire tread cap layer may be of an individual rubber composition designed to present a tread running surface for a balance of a combination of wet traction, cold weather winter traction (for snow and/or ice), dry handling, and resistance to tread wear properties.
However, optimizing one or more individual tread properties such as, for example, wet traction, cold weather winter traction, dry handling and resistance to tread wear properties typically requires a compromise of one or more physical properties.
Accordingly, for this invention, it is desired to present an outer tread cap layer with a running surface comprised of a plurality of individual circumferential load-bearing zones which exhibit one or more graduated physical properties, and which extend from the outer running surface of the tread cap layer radially inward to said tread base layer.
In practice, at least one, and preferably two primary tread cap zones are provided which constitute at least half of the tread running surface intended to be ground contacting. The remainder of such tread running surface is comprised of at least one and preferably at least two supplemental tread cap zones. The supplemental tread cap zone(s) are comprised of one or two lateral tread cap zones individually positioned axially outward of said primary tread cap zone(s) and/or a central tread cap zone wherein such central tread cap zone divides said primary tread cap zone into two primary tread cap zones.
For a symmetric tire tread cap, the tread cap may be comprised of the primary tread cap zone(s) and supplementary tread cap zone(s) as two lateral tread cap zones of substantially equal widths and/or a central tread cap zone.
For an asymmetric tire tread, at least one of said supplemental lateral tread cap zones may be provided of unequal widths in the case of two lateral tread cap zones and/or at least one of said primary tread cap zones may be provided of unequal widths in the case of two primary tread cap zones with a supplemental central tread cap therebetween. Accordingly, in such case, the central tread cap zone may not be centered over the centerline (the equatorial plane) and thereby may be in an off-centered position.
The rubber compositions of the strategically positioned rubber tread cap zones of the tread cap layer present a cooperative combination of graduated physical properties across the running surface of the tire in a sense of dynamic storage moduli (G′) at +60° C. and G′ at −25° C. and dynamic loss modulus (G″) at 0° C. for the individual tread cap zone rubber compositions.
In particular, the optional supplemental central tread cap zone has a dynamic storage modulus (G′) at −25° C. which is less than the storage modulus (G′) at −25° C. of said primary tread cap zone(s). A tire tread cap running surface is thereby presented where the optional central tread cap zone presents a tread cap running surface which is thereby relatively less stiff (e.g. somewhat softer) at lower temperatures than the associated primary tread cap zones to promote an accommodation of the tread for winter (snow and/or ice) driving conditions.
The supplemental lateral tread cap zone(s) have a storage modulus (G′) at 60° C. which is greater than the storage modulus (G′) at 60° C. of said primary tread cap zone(s). A tire tread cap running surface is thereby presented comprised of zones in which the lateral tread cap zone(s) are relatively stiffer at a 60° C. temperature than the associated primary tread cap zone(s) to promote an accommodation of the tread for non-winter driving conditions.
In practice, the lateral tread cap zone(s) have a loss modulus (G″) at 0° C. property which is greater than such loss modulus (G″) at 0° C. property of the said primary tread cap zone(s) to promote wet traction and wet handling.
It is therefore considered herein that it is a significant aspect of the invention for a tire tread cap layer comprised of said primary tread cap zone(s) and at least one supplemental tread cap zones (e.g. central and/or lateral tread cap zones), that the aforesaid storage moduli G′(at −25° C.) and G′ (at 60° C.) and loss modulus G″ (at 0° C.) physical properties relating to the said tread cap layer zones are combined in a cooperative manner to provide the overall tread running surface with suitable gradations of physical properties relating to the aforesaid wet traction, cold weather winter traction and/or non-winter handling.
Historically, tire treads have heretofore been suggested having running surfaces composed of three longitudinal portions namely, two black colored lateral portions and a non-black colored central portion located between the two black portions, wherein the lateral black colored portions have wear resistant properties virtually identical to the central colored portion (EP 0 993 381 A3, FR 2765525 and WO 99/01299 patent publications). It is considered herein that such revelation does not teach or suggest the strategically positioned and physical property gradation based tire tread cap zones of the present invention.
In U.S. Pat. No. 5,225,011 a tire is presented having a tread composed of a center rubber composition and side rubbers (its FIG. 1) positioned directly onto a tire carcass belt without a tread base transition layer. The center rubber is required to be limited to either natural rubber or a natural rubber/styrene-butadiene rubber blend. The center rubber contains a carbon black of large iodine absorption number of at least 100 mg/g, silica and silane coupling agent and the side rubbers are required to be of a different rubber composition. It is considered herein that such revelation does not teach or suggest the strategically positioned and physical property gradation based tire tread cap zones of the present invention.
In European patent publication number EP 864,446 A1 a tire is presented having a tread (its FIG. 2) with a central portion (B) and side portions (A) positioned directly onto a tire carcass belt without a tread base transition layer. The side portions are carbon black rich and the central portion is silica rich, wherein the silica content of the central portion (B) is at least 20 percent higher than in the side portions (A). It is considered herein that such revelation does not teach or suggest the strategically positioned and physical property gradation based tire tread cap zones of the present invention.
For the zoned tread cap layer of this invention, by requiring the tread cap zones to be capable of being load-bearing, it is meant that each of the distinct tread running surface tread cap zones extend radially inward from the outer surface of the tread cap layer to the underlying tread base layer rubber composition so that the load on the tire may be communicated by the tread cap layer zones to the transitional tread base layer instead of directly to remainder of the tire carcass itself.
In one aspect of this invention, the running surfaces of said optional supplemental central and said primary tread cap zone(s) are normally ground-contacting and the running surfaces of said optional, supplemental one or two, preferably two, tread cap zone(s) may preferably positioned to be ground contacting only during cornering conditions and, although continuing to be a part of the total running surface of the tire tread cap layer, would, in such case, be considered herein as being intermittently ground-contacting.
The term “running surface”, or “total running surface”, of the tread cap layer, unless otherwise indicated, means the total outer surface of such tread cap layer which is intended to be ground-contacting, including such outer surface of the tread cap layer which is intended to intermittently ground-contacting and the included space across the opening of any tread grooves contained in such tread cap layer at the running surface level. When a tread cap zone is referenced herein as spanning a percentage of total running surface of the tread cap, unless otherwise indicated, such span extends axially, or laterally, across such running surface (e.g. basically, in a direction substantially perpendicular to the equatorial plane of the tire).
In the description of this invention, the terms “rubber” and “elastomer” where herein, are used interchangeably, unless otherwise provided. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise provided. In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer.
The dynamic storage moduli (G′) and dynamic loss modulus (G″) viscoelastic properties of a cured rubber composition or tread sample are obtained using a ARES™-LS2 rheometer from the TA Instruments company of New Castle, Del. (USA) and equipped with a liquid nitrogen cooling device and forced convection oven to allow testing of rubber samples over a broad temperature range below and above ambient temperature. A cylindrical cured rubber sample is used which is approximately 8 millimeters in diameter and approximately 2 millimeters in height glued between two brass cylinders of approximately 8 millimeters in diameter. Such glue may, for example, be a cyanoacrylate based glue. The Orchestrator™ software was used to control the ARES™-LS2 rheometer. Using said software, the temperature increase rate of 5° C. is set. A temperature sweep at 3 percent torsional strain and 10 Hertz frequency of from, for example, about −30° C. to about +60° C. is used in which the dynamic storage moduli (G′) and dynamic loss modulus (G″) values may be determined simultaneously over said temperature range. From said determined dynamic storage moduli (G′), observations are made at −25° C. and +60° C. and from said determined loss moduli (G″), observations are made at 0° C. Use of dynamic storage modulus (G′) and dynamic loss modulus (G″) viscoelastic properties to characterize various aspects of cured rubber compositions is well known to those having skill in such art.